Common Heat Pump Problems and How to Fix Them

When a Heat Pump Goes Wrong, Start Calm

A heat pump that suddenly feels off can be alarming, especially in the middle of a cold snap. The good news is that most of the faults UK owners report are not faults at all. They are settings that have drifted, filters that need a clean, or a unit doing exactly what it is designed to do, such as defrosting on a frosty morning. A genuine mechanical failure is comparatively rare, and a well maintained heat pump should run for 15 to 25 years with only routine servicing.

This guide works through the problems people actually search for, roughly in the order you should approach them. We start with the five quick checks anyone can do safely, then move through the most common air source and ground source issues, the noises and leaks that worry people, and finally the line between what you can sort yourself and what needs a qualified engineer. Throughout, the aim is to help you diagnose the likely cause before you spend money on a call-out.

A short word of caution before we start. You can safely check thermostats, filters, isolators and the area around your outdoor unit. You should never open the sealed refrigerant circuit, touch electrical terminals, or attempt anything that requires F-Gas certification. Refrigerant handling in the UK is regulated under F-Gas regulations, and getting it wrong is both illegal and dangerous.

Quick Fixes: Five Things to Check Before Calling an Engineer

Before you assume the worst, run through these five checks. They resolve a large share of the "my heat pump has stopped working" reports, and they cost nothing.

Check the thermostat and controls. Confirm the system is set to heating mode, the target temperature is above the current room temperature, and any schedule or holiday mode has not switched the heating off. Smart thermostats such as Hive or a third-party app can quietly drop into eco or away mode after an update.
Inspect and clean the filters. Many systems have a strainer or magnetic filter on the heating circuit, and some have air filters on the indoor unit. A clogged filter restricts flow and saps performance. Cleaning or replacing filters every one to three months keeps the system breathing freely.
Clear the outdoor unit. Remove leaves, snow, tall grass and debris from around and beneath the fan. The unit needs free airflow, and a blocked grille forces it to work harder for less heat.
Check the power supply. Look in your consumer unit for a tripped breaker or RCD, and confirm the dedicated isolator near the outdoor unit is switched on. A nuisance trip after a power cut is common and easily reset.
Check system pressure. Most wet heating systems should sit between 1 and 1.5 bar when cold. If the gauge reads near zero, the system may have lost water and shut down on low pressure protection. Topping up is usually straightforward, but a repeated drop points to a leak worth investigating.

If all five check out and the system still misbehaves, the sections below will help you narrow down the cause.

Heat Pump Not Heating Properly: Causes and Fixes

A house that never quite warms up is the single most common complaint, and it usually has nothing to do with a broken heat pump. More often the system was set up to behave like a gas boiler, which is the wrong model entirely.

The most frequent cause is a flow temperature or weather compensation curve set too low for the property. A heat pump runs best at a low, steady flow temperature, but if the heating curve is flatter than the building's heat loss demands, the radiators simply cannot deliver enough heat on the coldest days. The fix is a curve adjustment during or after commissioning, ideally by your installer, who can raise the design-point flow temperature without wrecking efficiency.

The second common cause is undersized emitters. If your radiators were sized for a 70C boiler and were never upgraded, they may not give out enough heat at a 40C to 45C flow temperature. This is not a heat pump fault, it is a design mismatch. Our guide on running a heat pump with radiators explains how to tell whether your existing radiators are up to the job and which rooms tend to fall short first.

Other causes worth ruling out include a heating schedule that switches off too aggressively overnight, a hot water priority cycle stealing heating time, air trapped in the system after a top-up, and a circulation pump running too slowly. If none of these explain it, and the flow temperature reads correctly but the return temperature barely changes, a qualified engineer should check the refrigerant charge and the compressor.

A Note on Cold Weather

People often blame the heat pump when temperatures plunge, but a correctly sized system should keep a UK home warm well below freezing. Most modern air source units are rated to operate down to minus 15C to minus 20C, albeit at reduced efficiency. If your home only struggles in genuinely bitter weather, the issue is more likely insulation, emitter sizing or curve settings than the heat pump itself. Our guide to heat pump winter performance covers what is normal and what is not when the mercury drops.

Why Your Outdoor Unit Ices Up

A layer of frost or ice on the outdoor unit in winter looks alarming, but in most cases it is completely normal. As the unit extracts heat from cold, damp air, condensation forms on the cold evaporator coil and freezes. Every modern heat pump runs a defrost cycle to clear this, briefly reversing operation to warm the coil and melt the ice. You may notice steam, a puddle of meltwater, or the heating pausing for a few minutes. This is the system working correctly, not failing.

When icing becomes a genuine problem is when the unit stays encased in ice, the defrost cycle never seems to clear it, or ice builds up underneath the unit because meltwater cannot drain away. The usual culprits are a blocked or frozen condensate drain, the unit mounted too low to the ground, or a defrost sensor or control fault.

What you can safely do: keep the area beneath and around the unit clear so meltwater drains freely, gently remove heavy snow from the top of the unit by hand, and make sure the unit sits on feet or a stand that lifts it clear of the ground. What you must never do: pour hot water over the coil, chip at ice with a tool, or use a heat gun, all of which can damage the fins or the refrigerant circuit. If the unit stays frozen solid despite a clear drain and good airflow, the defrost control needs an engineer.

Unusual Noises: What They Mean

A heat pump is not silent, but it should produce a steady, low hum rather than dramatic sounds. Modern units typically operate around 40 to 50 dB at one metre, roughly the level of a quiet conversation or a fridge. A sudden change in the noise it makes is worth investigating.

Gurgling or bubbling usually means air trapped in the heating circuit. Bleeding radiators and checking system pressure often cures it. Persistent gurgling can indicate low refrigerant, which is an engineer job.
Rattling or vibration is frequently a loose panel, a fan grille, or the unit's anti-vibration mounts working loose. Tightening fixings and checking the mounting feet usually solves it.
Clicking at the start and end of a cycle is normal relay and valve operation. Continuous clicking is not.
Hissing or whooshing during a defrost cycle is the reversing valve and is normal. Constant hissing can point to a refrigerant leak.
Loud humming or grinding from the fan or compressor suggests a bearing or mechanical fault, and the system should be checked promptly.

If the noise is more of a neighbour relations issue than a fault, planning rules and placement matter. Our guide on heat pump noise and neighbours covers permitted development limits and how to site a unit to keep sound levels down.

Leaks and Pressure Loss

There are two very different kinds of leak, and it is important not to confuse them.

A water leak from the heating side shows up as dripping pipework, damp patches, or a slowly falling pressure gauge that needs frequent topping up. Common points are valve glands, pump seals, and joints that have worked loose. A small weep can sometimes be tightened, but a persistent loss should be traced and sealed before it causes corrosion or damp. Constantly topping up fresh, oxygenated water into the system also accelerates internal corrosion and sludge, so chasing the leak matters.

A refrigerant leak is a different matter entirely. Symptoms include falling heat output, the unit running constantly without warming the house, hissing noises, and sometimes more icing than usual. Refrigerant is sealed in the circuit and should never need topping up under normal operation, so a low charge always means a leak. Because refrigerant is regulated under F-Gas rules, only a certified engineer may diagnose and repair it. Do not attempt this yourself.

Finally, the condensate drain. Heat pumps produce condensate, especially during defrost, and this needs to drain away. A blocked or frozen condensate pipe can cause pooling water, icing problems, and in some systems a fault shutdown. Keeping the drain clear is a simple owner task that prevents several knock-on problems.

Short Cycling: When the System Keeps Stopping and Starting

Short cycling is when the heat pump switches on and off frequently rather than running steadily. It is inefficient, wears the compressor, and often leaves the house feeling uneven. A heat pump is happiest modulating gently and running for long stretches, so frequent cycling is a sign something is forcing it to stop.

The usual causes are an oversized heat pump for the property's heat loss, which means it satisfies demand too quickly and then stops, or too little water volume in the system to absorb the heat, which is why many installations include a buffer tank or volumiser. Other triggers include a thermostat with too tight a switching differential, a partially closed valve, or a circulation pump set too low. Settings adjustments often help, but a genuinely oversized unit or a system short of volume usually needs an installer to assess and correct.

Air Source vs Ground Source: How the Problems Differ

The two main heat pump types share many issues but differ in a few telling ways. This quick comparison helps you focus your diagnosis.

Symptom or issue	Air source (ASHP)	Ground source (GSHP)
Icing on outdoor unit	Common in winter, normal during defrost	Not applicable, no exposed evaporator
Performance drop in cold weather	More noticeable, air is the heat source	More stable, ground stays warmer
Outdoor fan noise	Possible, fan and compressor outdoors	Minimal, no outdoor fan unit
Refrigerant leak	Possible, requires F-Gas engineer	Possible, requires F-Gas engineer
Low loop or ground array pressure	Not applicable	Possible, ground loop can lose pressure
Antifreeze or brine issues	Not applicable	Brine concentration can need checking

Ground source systems remove the icing and outdoor fan noise problems entirely, but introduce ground loop considerations such as brine pressure and antifreeze concentration, which are checked at service. Air source systems are far more common in the UK, which is why most troubleshooting content focuses on them.

How Poor Insulation Masquerades as a Heat Pump Problem

A surprising number of "heat pump problems" are really building problems. A heat pump delivers heat at a low, steady rate, which works beautifully in a home that holds onto warmth and poorly in one that leaks it. If your house feels cold despite the system running correctly, the fabric of the building may be the real bottleneck.

A poorly insulated home loses heat faster than a low-temperature system can replace it, which can leave rooms feeling cool on the coldest days even though nothing is mechanically wrong. The same home also reheats slowly and expensively after any set-back. In these cases, no amount of heating curve tweaking fully solves the comfort problem, because the heat is escaping as fast as it goes in.

The Energy Saving Trust consistently emphasises that insulation and heat pumps work as a pair. Loft insulation, cavity wall insulation, draught proofing and better glazing all reduce the heat your home needs, which lets the heat pump run at a lower flow temperature, more efficiently, with fewer comfort complaints. If you are diagnosing a stubborn warmth problem, it is always worth asking whether the building, rather than the boiler, is the weak link.

When to DIY and When to Call a Professional

Knowing where the line sits saves you both money and risk. Here is a clear split.

Tasks you can safely do yourself:

Checking and adjusting thermostat and schedule settings
Cleaning or replacing accessible filters and strainers
Clearing leaves, snow and debris from around the outdoor unit
Bleeding radiators and topping up system pressure to the correct level
Resetting a tripped breaker or isolator after a power cut
Keeping the condensate drain clear

Tasks that need a qualified engineer:

Anything involving the refrigerant circuit, which requires F-Gas certification
Electrical faults beyond a simple reset
Compressor, fan motor or reversing valve problems
Refrigerant leaks, recharging or persistent low charge
Recurring faults, error codes you cannot clear, or any safety concern
Weather compensation curve recommissioning if you are not confident

A typical heat pump repair in the UK costs between £150 and £600 depending on the fault, with simple sensor or control jobs at the lower end and compressor work at the higher end. Many issues are covered under warranty if the unit is still within term and has been serviced as required, so always check your warranty before paying for a repair. Our guide to heat pump maintenance costs breaks down servicing prices and what regular maintenance includes.

When you do need a professional, use an installer who is certified under the Microgeneration Certification Scheme (MCS) and ideally trained on your specific brand, whether that is Daikin, Mitsubishi Electric, Vaillant or another manufacturer. You can find verified, rated installers through our installer finder, which lists MCS-certified engineers who can diagnose and repair your system properly. Using an MCS-registered installer also protects any eligibility you have under the Boiler Upgrade Scheme and keeps your warranty intact.

Preventing Future Problems

Most heat pump faults are easier to prevent than to fix. A few habits keep the system reliable for the long haul.

Book an annual service with a qualified engineer, who will check refrigerant pressure, electrical connections, the defrost system and overall performance, catching small issues before they become breakdowns. Between services, keep filters clean, the outdoor unit clear, and the condensate drain free. Make sure your weather compensation curve is properly set during commissioning, because a badly tuned curve is behind a large share of comfort and efficiency complaints. And resist the urge to treat your heat pump like a boiler with deep overnight set-backs, which forces inefficient hard recovery and can look like a fault.

Finally, keep your service records. They are evidence for warranty claims, they help an engineer spot a gradual performance decline, and they make selling the property with a documented, well kept system far easier.

Frequently Asked Questions

Why is my heat pump running constantly but the house is not warm? This usually points to a flow temperature or heating curve set too low, undersized radiators, a refrigerant leak, or simply a very cold day where the system cannot keep pace because of insulation losses. Check the flow temperature and emitters first, and if the unit runs flat out without warming the house, have the refrigerant charge checked.

Is it normal for my heat pump to ice up in winter? Yes. Frost on the outdoor coil is normal, and the unit runs a defrost cycle to clear it. It only becomes a problem if the unit stays encased in ice, the defrost never clears it, or meltwater cannot drain away.

How much does a heat pump repair cost in the UK? Most repairs fall between £150 and £600 in 2026, depending on the fault. Sensor and control jobs sit at the lower end, while compressor or major component work is more expensive. Many faults are covered under warranty if the system is within term and has been serviced as required.

Can I fix heat pump problems myself? You can safely check thermostats, clean filters, clear the outdoor unit, top up pressure and reset breakers. Anything involving the refrigerant circuit, electrical faults or major components requires a qualified, F-Gas certified engineer.

How long should a heat pump last? A well maintained heat pump typically lasts 15 to 25 years, considerably longer than a gas boiler. Annual servicing and keeping filters and the outdoor unit clear are the main factors in reaching the upper end of that range.